What to make flexible

In the book Universal Principles of Design by William Lidwell, Kritina Holden, and Jill Butler, the authors have this to say about the flexibility-usability trade-off.

It is a common assumption that designs should always be made as flexible as possible. However, flexibility has real costs in terms of complexity, usability, time, and money; it generally pays dividends only when an audience cannot clearly anticipate its future needs.

Good design is not about making everything flexible. It’s about making the right things flexible, and making other things more rigid. Good design could be described in terms of what is rigid as much as in terms of what is flexible. The art is knowing which things to make flexible, while casting other things in stone.